Method for activating solid polymer fuel cell

ABSTRACT

By using a fuel cell having a membrane electrode assembly having and electrolyte membrane  100  formed by a polymer electrolyte membrane, and an anode  101  and a cathode  102  carrying a catalytic metal and sandwiching electrolyte membrane  100 . The anode  101  and the cathode  102  are in electrical connection, and an activation treatment is carried out for opening an active site of catalytic metal of the cathode  102 . This provides an activation method of a polymer electrolyte fuel cell which is advantageous to activation and raises cell voltage.

This application is a 371 of PCT/JP04/07149, filed 19 May 2004 whichclaims priority from Japanese application 2003-143126, filed 21 May2003.

TECHNICAL FIELD

The present invention relates to an activation method of a polymerelectrolyte fuel cell by activating a fuel cell to raise cell voltage.

BACKGROUND ART

Conventionally, before a fuel cell is put into normal operation, therehas been carried out a method in which the current with higher currentdensity than the predetermined current density is passed to raise anoutput of the fuel cell, namely—pre-running. The effect of raising unitvoltage can be obviously achieved when pure oxygen gas for working as anoxidant is supplied to a cathode, and when pre-running is carried outwith the current density as much as possible, and the duration as longas possible.

However, the above-mentioned pre-running method is operationallycomplicated and time-consuming. Besides, as the heavy current running isneeded, it causes flooding and massive heat, both of which may damage anelectrolyte membrane. The so-called flooding refers to the phenomenon ofthe blockage of the cathode flow way by the water generated on thecathode. Due to the existence of such an unfavorable factor, the use ofthe above-mentioned method can hardly obtain an inherent capacity of thefuel cell itself effectively.

Patent Literature No. 1 discloses the activation technique of waterelectrolysis. This technique pays attention to the fact that in a fuelcell having an ion exchange membrane formed by a polymer electrolytemembrane, and a unit cell including an anode and a cathode forsandwiching the ion exchange membrane, and separators, the fuel cell isactivated by increasing water content of the ion exchange membrane. Inthis technique, while the humidified gas is supplied to the cell, 1.3 vor more of electrolytic voltage is applied to the cell for waterelectrolysis. According to Patent Literature No. 1, water in anelectrolyte membrane is compulsively decomposed into hydrogen and oxygenby the electrolysis, and in accordance with this, the concentrationgradient of water molecules in the electrolyte membrane increases, anddue to this, the water diffusion speed in the electrolyte membraneincreases. As a result, water in the humidified gas flows to theelectrolyte membrane to rapidly increase the water content.

Patent Literature No. 2 discloses the technique of restoring thedegradation of a fuel cell. This technique pays attention to the factthat when metal ions such as iron, nickel, and the like are trapped intoa high polymer electrolyte membrane, the ionic conductivity of the highpolymer electrolyte membrane decreases and the power generationperformance is weakened. In this technique, when a fuel cell isdegraded, a fluid for restoring degradation which contains a reductantfor restoring degradation (hydrazine or hydrazine salt) with strongerreducing power than that of hydrogen is brought into contact with thehigh polymer electrolyte membrane, so as to restore the degradation ofthe power generation performance of the fuel cell, whose degradation iscaused by the metal ions adhered to the high polymer electrolytemembrane. According to Patent Literature No. 2, the metal ions such asiron, nickel and the like, which are trapped into the high polymerelectrolyte membrane, which are reduced by the reductant for restoringdegradation with stronger reducing power, and which is precipitated asmetals. Thus, the metal ions are removed, and the power generationperformance is enhanced.

In addition, Patent Literature No. 3 discloses the technique ofactivating a high polymer electrolyte membrane fuel cell, in whichmodules of the high polymer electrolyte membrane of a fuel cell isboiled in deionized water or mild acid water (such as hydrogen peroxidewater). Besides, Patent Literature No. 3 discloses the technique ofactivating a high polymer electrolyte membrane fuel cell, in whichalcohol is supplied to a gas supply way of the high polymer electrolytemembrane fuel cell to adapt an electrode diffusion layer to alcohol.Patent Literature No. 3 also discloses the technique of activating ahigh polymer electrolyte membrane fuel cell, in which the powergeneration of modules of the high polymer electrolyte membrane fuel cellis carried out with oxygen utilization rate of 50% or more, and theaverage cell voltage is kept to be 0.3 v or less.

(Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI)No. 6-196, 187)

(Patent Literature No. 2: Japanese Unexamined Patent Publication (KOKAI)No. 2000-260,453)

(Patent Literature No. 3: Japanese Unexamined Patent Publication (KOKAI)No. 2000-3718)

However, according to the above-mentioned patent literatures 1-3,procedures are complicated, and time-consuming, and besides, floodingand massive heat caused by the heavy current running, may damage theelectrolyte membrane. Since these defects exist, in the above-mentionedmethod, it is not necessarily clear that an inherent capacity of thefuel cell itself is obtained.

The present invention is made, taking account of the above-mentionedcircumstances. An object of the present invention is to provide anactivation method of a polymer electrolyte fuel cell which isadvantageous to activation and raises cell voltage.

DISCLOSURE OF THE INVENTION

(1) The present inventor has progressed analytical study on activity ofa fuel cell by an electrochemical AC (alternating current) impedancemethod by using an impedance analyzer which is called as a frequencyresponding analytical device and a polymer electrolyte fuel cell. Here,the fuel cell has membrane electrode assemblies which are formed inmulti-layered and each of which has an electrolyte membrane formed by apolymer electrolyte membrane, and an anode and a cathode carrying acatalytic metal to sandwich the electrolyte membrane. Theelectrochemical AC impedance method is a model test which is carried outby an equivalent circuit in which an electrochemical reaction issubstituted with an electrical circuit.

The following shows typical Examples of making analysis which uses ACimpedance method in the demonstration test. In this case, pure hydrogengas (pressure: normal pressure) is supplied to the anode, and air(pressure: normal pressure) is supplied to the cathode. FIG. 1 indicatesthe relation among the time, the cell voltage and the current densitywhen the power generation running of a fuel cell starts, and pre-runningwhich is conventionally carried out by the present inventor. In FIG. 1,the characteristic lines V1, V2 and V3 indicate the cell voltage and thecharacteristic lines A1, A2 and A3 indicate the current density. Thecharacteristic line V1 indicates the voltage characteristic fromstarting the power generation running of the fuel cell to starting thepre-running. The characteristic line A1 indicates the currentcharacteristic from starting generation running of the fuel cell tostarting the pre-running. The characteristic line V2 indicates thevoltage characteristic during the pre-running. The characteristic lineA2 indicates the current characteristic during the pre-running. In thispre-running, as illustrated in the characteristic line A2, the heavycurrent, having a current density of 0.5 A/cm², flows. Thecharacteristic line V3 indicates the voltage characteristic after thepre-running. The characteristic line A3 indicates the currentcharacteristic after the pre-running.

As shown in the characteristic line V1 of FIG. 1, the cell voltagegradually increases in accordance with starting of the power generationrunning of the fuel cell. When the cell voltage increases to a certainsaturated state (point 2), the conventional pre-running begins. In thepre-running, a current density of heavy current (0.5 A/cm²) is adopted,as shown in the characteristic line A2. At this time, the unit voltagedescends to about 0.60 v, as shown in the characteristic line V2.

As the pre-running progresses, as shown in the characteristic line V2,the unit voltage begins to increase gradually from around 0.60 v. Whenthe conventional pre-running is finished, as shown in the characteristicline V3, the cell voltage can be restored and increased higher than thatof just before the pre-running, thereby achieving the activation effect.Thus, it is apparent that making lower contemporary of the cell voltageis effective for activating the fuel cell.

In FIG. 1, point 1 indicates the condition after the starting of thepower generation running of the fuel cell, point 2 indicates thecondition just before the pre-running of the fuel cell, and point 3indicates the condition just after the pre-running of the fuel cell. Thepresent inventor analyzes on points 1, 2 and 3 by an electrochemical ACimpedance method. FIG. 2 indicates the analytical result (Cole-ColePlot) by the AC impedance method, which is indicated as the complexplane. The electrochemical impedance Z is expressed as the complexquantity containing a real number Re and an imaginary number Im in thefollowing formula (1).Z(impedance)=R+j Im  (1)

The horizontal axis of FIG. 2 indicates the real component of impedance,and the vertical axis of FIG. 2 indicates the imaginary component ofimpedance. The “5.00E−03” on the horizontal axis indicates 5.00×10⁻³;the “−5.00E−03” on the longitudinal axis indicates −5.00×10⁻³. As shownin FIG. 2, in point 1 just after the pre-running of the fuel cell, thecell resistance including the resistance of the electrolyte membrane isequivalent to R₁₁−R₀; the resistance of the electrode reaction isequivalent to R₂₁−R₁₁; the resistance of the electrode reaction isrelatively higher. As shown in FIG. 2, in point 2, just before thepre-running of the fuel cell, the cell resistance is equivalent toR₁₂−R₀, the resistance of the electrode reaction is equivalent toR₂₂−R₁₂, the resistance of the electrode reaction is lower than that inpoint 1, just after the pre-running. It can be inferred that the watercontent of the membrane increases gradually after the pre-runningbegins. In addition, as shown in FIG. 2, in point 3, just after thepre-running of the fuel cell, the cell resistance is equivalent toR₁₃−R₀, and the resistance of the electrode reaction is equivalent toR₂₃−R₁₃, and the cell resistance almost remains unchanged in comparisonwith point 2. However, the resistance of the electrode reaction isreduced by ΔR in comparison with point 2, just before the pre-running.Thus, it is favorable for raising the output of the fuel cell.

Through the above-mentioned analytical result as shown in FIG. 1 andFIG. 2, the present inventor finds that, the impregnation effect itselfof the soaked electrolyte membrane is not enough, though it is effectivefor activation. The potential voltage of the cathode should be loweredas much as possible in activation treatment. That is, it should be asmuch as close to the standard electrode potential 0 v of theoxidation-reduction system of hydrogen, which is more favorable foractivation of the fuel cell, and the cathode potential can be moreeasily restored than before the activation treatment. The cathodepotential can be easily restored in comparison with before activationtreatment when the cathode potential is lowered as much as possible inactivation treatment. This reason has not been clarified, but it can beinferred that the electrochemical reduction reaction of oxygen workingas an active material on the cathode is more restricted than beforeactivation treatment, thus decreasing the cathode potential to promoteother electrochemical reduction reactions (of the catalytic metal oxideand the adsorbed elements on the surface of the catalyst) on thecathode.

(2) The characteristics of the activation method of polymer electrolytefuel cell according to a first aspect of the invention are as follows:an activation method of a polymer electrolyte fuel cell having amembrane electrode assembly having an electrolyte membrane formed by apolymer electrolyte membrane, and an anode and a cathode carrying acatalytic metal and sandwiching the electrolyte membrane, wherein theanode and the cathode are in electrical connection, and an activationtreatment is carried out for opening an active site of the catalyticmetal of the cathode. It can be inferred that the activation treatmentpromotes other electrochemical reduction reaction (of the catalyticmetal oxide and the adsorbed elements on the surface of the catalyst) ofthe cathode. Thus, the active sites of the catalytic metal on thecathode are opened, and the catalytic metal of the cathode is activated,and the reaction resistance of the cathode is lowered.

(3) The characteristic of the method of activating the polymerelectrolyte fuel cell according to a second aspect of the invention isas follows: an activation method of a polymer electrolyte fuel cellhaving a membrane electrode assembly having an electrolyte membraneformed by a polymer electrolyte membrane, and an anode and a cathodecarrying a catalytic metal and sandwiching the electrolyte membrane,wherein the anode and the cathode are in electrical connection, and anactivation treatment in which gas containing hydrogen is supplied to theanode and gas containing oxygen is supplied to the cathode, andpotential of the cathode is maintained at 0.5 v or less, to lowerresistance of the electrode reaction, is carried out. The anode refersto the electrode where the electrochemical oxidation reaction occurs,and the cathode refers to the electrode where the electrochemicalreduction reaction occurs. To maintain the cathode potential at 0.5 v orless means to take a potential on the basis of the standard electrodepotential of the oxidation-reduction system of hydrogen as the 0 v. Theupper limit value of the cathode potential in activation treatment canbe 0.4 v, 0.3 v, 0.2 v, and 0.1 v, for example; the lower limit value ofthe cathode potential in activation treatment can be −1.0 v, −0.5 v,−0.1 v, −0.05 v, −0.005 v, and +0.002 v, for example. A potentiostatapparatus can be used to keep the cathode potential voltage at theabove-mentioned level. The potentiostat apparatus is the device forapplying current to keep a constant potential voltage between the twoelectrodes.

According to the activation method of polymer electrolyte fuel cell, inthe second aspect of the invention, the analytical result obtainedthrough AC impedance method shows that the reaction resistance of theelectrode reaction decreases. Therefore, as shown in the after-mentionedexperimental Example 1, after the activation treatment is carried out,the active sites of the cathode catalytic metal are opened, and thecatalytic metal is activated. In power generation, the power generationvoltage of the fuel cell is higher than that before the activationtreatment.

In the activation method of the polymer electrolyte fuel cell in thesecond aspect of the invention, the activation treatment can be carriedout just before the normal power generation running of the fuel cell isstarted, or after the normal power generation running temporarily stopswhen the unit voltage of the fuel cell is found descend, or in theprocess of power generation running. To be carried out during powergeneration running means that the activation method is carried out inthe state of unceasing current output of the fuel cell. To be carriedout after the generation running stops means that the activation methodis carried out when the output current of the fuel cell stops.

According to the activation method of the polymer electrolyte fuel cellin the second aspect of the invention, it is possible to exemplify amode in which a second conductive path whose electric resistance isrelatively lower than that of a first conductive path in normaloperation for generation of electricity. In this case, the activationtreatment can be carried out in the condition that the anode and thecathode are in electrical connection by way of the second conductivepath with a relatively lower resistance, instead of through the firstconductive path which is used in the normal operation. In thissituation, in the activation treatment, electrons (e⁻) generated in theelectrochemical oxidation reaction on the anode move to the cathodethrough the second conductive path with a relatively lower resistance,instead of through the first conductive path having a relatively higherresistance. Therefore, it is favorable for accelerating a speed of theactivation treatment of the cathode.

(4) The characteristic of the method of activating the polymerelectrolyte fuel cell according to a third aspect of the invention is asfollows: an activation method of a polymer electrolyte fuel cell havinga membrane electrode assembly having an electrolyte membrane formed by apolymer electrolyte membrane, and an anode and a cathode carrying acatalytic metal and sandwiching the electrolyte membrane, wherein theanode and the cathode are in electrical connection, and an activationtreatment is carried out by supplying gas containing hydrogen to theanode and supplying non-oxidant gas to the cathode.

The non-oxidant gas can be inert gas (nitrogen gas, argon gas and so on)or hydrogen gas, or the mixture of these gases. According to theactivation method of the polymer electrolyte fuel cell in the thirdaspect of the invention, it can be inferred that, as the gas containinghydrogen is supplied to the anode in the condition that the anode andthe cathode are in electrical connection, hydrogen is decomposed intoprotons (H⁺) and electrons (e⁻) by electrochemical oxidation reaction onthe anode, the electrons (e⁻) move to the cathode by the connection, andthe electrons (e⁻) are applied to electrochemical reduction reaction onthe cathode.

In addition, in activation treatment, non-oxidant gas, namely, the inertgas such as nitrogen gas, argon gas and so on, hydrogen gas and so on,the mixture of these gases (purge gas), which plays the role of theactivating gas, which gas is supplied to the cathode, so that an oxygenshortage condition is actively generated on the cathode during theactivation treatment. Therefore, it can be inferred that theelectrochemical reduction reaction of other substances is conducted moreactively on the cathode than the electrochemical reduction reactionconcerning oxygen molecules. That is, in the stages of manufacturing thefuel cell, placing it aside, and making power generation, it is inferredthat the product such as oxide and so on is generated or substances areadsorbed on the surface of the catalytic metal of the cathode. In theactivation treatment, the electrochemical reduction reaction in relationto the catalytic metal occurs, and the oxide and adsorbed elements onthe catalytic metal are removed, so that the active sites of the cathodecatalytic metal are opened, to activate the catalytic metal, and thereaction resistance of the electrode reaction on the cathode is lowered.

In the above-mentioned activation method, as the non-oxidant gas (purgegas) such as nitrogen gas and so on is supplied to the cathode, the gasfunctioned as the activating gas can be distributed evenly to each ofthe cells of the cathode even when the number of cells constituting thefuel cell is plural, so as to decrease the uneven activation treatmentbetween all the cells.

According to the activation method of the polymer electrolyte fuel cellin the third aspect of the invention, as shown in the after-mentionedExample 2, the power generation voltage of the fuel cell after theactivation treatment is raised as compared with before the activationtreatment. According to the activation method of the polymer electrolytefuel cell in the third aspect of the invention, the cathode potentialcan be 0.5 v or less. Due to this, the activation effect can be furtherpromoted. Furthermore, in this method, the upper limit value of thecathode potential can be 0.4 v, 0.3 v, 0.2 v, and 0.1 v. In activationtreatment, depending on conditions, the lower limit value of the cathodepotential can be −1.0 v, −0.5 v, −0.1 v, −0.05 v, −0.005 v, and +0.002v. In the activation method of the polymer electrolyte fuel cell in thethird aspect of the invention, the activation treatment can be carriedout just before the normal power generation running of the fuel cell isstarted, or after the normal generation running temporarily stops whenthe unit voltage of the fuel cell is found descend. However, theactivation method of the polymer electrolyte fuel cell in the secondaspect of the invention can be carried out after the manufacturingbefore shipping of the fuel cell or in the process of power generation.

(5) The characteristic of the method of activating the polymerelectrolyte fuel cell according to a fourth aspect of the invention isas follows: an activation method of a polymer electrolyte fuel cellhaving a membrane electrode assembly having an electrolyte membraneformed by a polymer electrolyte membrane, and an anode and a cathodecarrying a catalytic metal and sandwiching the electrolyte membrane,wherein the anode and the cathode are in electrical connection, and anactivation treatment is carried out by supplying gas containing hydrogento the anode and making the cathode an oxygen shortage condition.

According to the activation method of the polymer electrolyte fuel cellin the fourth aspect of the invention, it can be inferred as follows:The gas containing hydrogen is supplied to the anode in the conditionthat the anode and the cathode are in electrical connection. Therefore,hydrogen is decomposed into protons (H⁺) and electrons (e⁻) byelectrochemical oxidation reaction on the anode, and the electrons (e⁻)move to the cathode by the connection, and are applied toelectrochemical reduction reaction on the cathode. According to theactivation treatment, because the oxygen shortage condition is forciblymaintained on the cathode, the electrochemical reduction reactionconcerning oxygen is conducted on the cathode, and at the same time, theelectrochemical reduction reactions of other substances are conductedactively. That is, in the stages of manufacturing the fuel cell, placingit aside, and making power generation, it is thought that the productsuch as oxide and so on is generated or substances are adsorbed on thesurface of the catalytic metal of the cathode. In the activationtreatment, the electrochemical reduction reaction concerning thecatalytic metal occurs, and the oxide and adsorbed elements on thecatalytic metal are removed, so that the active sites of the cathodecatalytic metal are opened to activate the catalytic metal and thereaction resistance of the electrode reaction on the cathode is lowered.

According to the fourth aspect, the activation treatment can be carriedout under the oxygen shortage condition where the oxygen utilizationrate is over 100%. The oxygen utilization rate can be over 120%, over150% and over 200%. The oxygen shortage condition is enhanced bylowering the oxygen concentration in the gas to be supplied to thecathode, and the non-oxidant gas not containing oxygen can be used underthe limit condition. In this state, the oxygen utilization rate isinfinite, and there is no upper limit of oxygen utilization rate.

According to the activation method of the polymer electrolyte fuel cellin the fourth aspect of the invention, as shown in Example 3, the powergeneration voltage of the fuel cell after the activation treatment israised as compared with before the activation treatment. The activationtreatment can be carried out just before the normal power generationrunning of the fuel cell is started, or after the normal powergeneration running temporarily stops when the unit voltage of the fuelcell is found descend. Besides, the activation treatment can be carriedout after the manufacturing before shipping of the fuel cell, or in theprocess of power generation. According to the activation method of thepolymer electrolyte fuel cell in the fourth aspect of the invention, thecathode potential can be 0.5 v or less. Due to this, the activationeffect can be further promoted. According to the activation method ofthe polymer electrolyte fuel cell in the fourth aspect of the invention,in activation treatment, the upper limit value of the cathode potentialcan be 0.4 v, 0.3 v, 0.2 v, and 0.1 v, and the lower limit value can be−1.0 v, −0.5 v, −0.1 v, −0.05 v, −0.005 v, and +0.002 v.

(6) Accordance to the activation method of the polymer electrolyte fuelcell in the first to the fourth aspect of the invention, when thecathode potential is lower than that of the anode in activationtreatment, the electrons supplied from the anode to the cathode arelimited. Therefore, the activation treatment can be carried out byforcibly supplying electrons from the external apparatus (the externalenvironment) to the cathode. In this case, the active sites of thecathode catalytic metal are opened, and the cathode catalyst isactivated. The external apparatus can be another fuel cell, a capacitorand the like.

EFFECT OF THE INVENTION

The present invention provides an activation method of the polymerelectrolyte fuel cell which is advantageous for activation and can raisecell voltage. It can be inferred that the active sites of the catalyticmetal of the cathode can be opened by the activation treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the curve diagram indicating the transition of cell voltageand current density before, in, and after pre-running.

FIG. 2 is the curve diagram of the analytical result with AC impedancemethod, and shows the transition of the electrolyte membrane resistanceand the electrode reaction resistance before, in, and after pre-running.

FIG. 3 is the curve diagram indicating the transition of the cellvoltage and current density according to the third aspect, before and inpre-running, and in and after activation treatment.

FIG. 4 is the curve diagram indicating the analytical result with ACimpedance method, and shows the transition of the electrolyte membraneresistance and the electrode reaction resistance before, and afterpre-running and after activation treatment according to the thirdaspect.

FIG. 5 is the curve diagram indicating the transition of the unitvoltage and current density according to the fourth aspect, before, inand after activation treatment.

FIG. 6 is the curve diagram indicating the analytical result with ACimpedance method, and shows the transition of the electrolyte membraneresistance and the electrode reaction resistance in pre-running and theactivation treatment according to the fourth aspect.

FIG. 7 is the cell structure diagram according to Example 1.

FIG. 8 is the cell structure diagram according to Example 2.

FIG. 9 is the cell structure diagram modified in another form accordingto Examples 1, 2 and 3.

FIG. 10 is the cell structure diagram modified in other forms accordingto Examples 1, 2 and 3.

FIG. 11 is the structure diagram of power generation system of the fuelcell according to applicable Examples.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment A

According to Embodiment A, in a series of power generation running,pre-running equivalent to the prior art and an activation treatmentequivalent to the third aspect of the invention (non-oxidant gas wasintroduced into a cathode when the activation treatment) were carriedout. It is shown in FIG. 3. In the pre-running equivalent to the priorart, pure hydrogen gas (pressure: normal pressure) was supplied to ananode, and at the same time, air (pressure: normal pressure) wassupplied to a cathode, and the current density was increased to a heavycurrent of 0.5 A/cm² (characteristic line A6). According to EmbodimentA, in the activation treatment equivalent to the third aspect of theinvention, in the condition that the anode and cathode were inelectrical connection, the pure hydrogen gas (pressure: normal pressure)was supplied to the anode, and nitrogen gas for working as non-oxidantgas (pressure: normal pressure) was supplied to the cathode, and thecurrent density was set to be 0.38 A/cm² (characteristic line A8).

In FIG. 3, characteristic lines V5-V9 indicate voltage, andcharacteristic lines A5-A9 indicate current. The characteristic lines A5and V5 in FIG. 3 indicate the condition that the power generationrunning of the fuel cell starts. The characteristic lines A6 and V6indicate the condition that the pre-running equivalent to the prior artis carried out after the starting of the power generation running. Theplot of “◯” as shown in FIG. 3 is the mark of making Cole-Cole Plot. InEmbodiment A, the characteristic lines A7 and V7 in FIG. 3 indicate thecondition that the power generation running is carried out after theconventional pre-running. In Embodiment A, the characteristic lines A8and V8 in FIG. 3 indicate the condition that the activation treatmentequivalent to the third aspect of the invention is carried out. When theactivation treatment equivalent to the third aspect of the invention iscarried out, as shown in the characteristic line A8, the current densityis set at 0.38 A/cm², and as shown in the characteristic line V8, thecell voltage is set around 0 v in the plus range (about 0.005 v) Sincethe cell voltage is the potential of the difference between the cathodeand the anode and the voltage of the anode is regarded as 0 v, the cellvoltage will substantially be the potential of the cathode in fact.

In Embodiment A, the characteristic lines A9 and V9 in FIG. 3 indicatethe condition that the normal power generation running is carried outafter the activation treatment equivalent to the third aspect of theinvention. As understood from a comparison between the characteristicslines V7 and V9 in FIG. 3, it confirmed that when the activationtreatment equivalent to the third aspect of the invention (in which thenon-oxidant gas was introduced into the cathode when the activationtreatment) was carried out, the voltage was ΔVb higher than that of thepre-running equivalent to the prior art, thereby achieving theactivation effect. Furthermore, according to the experiment shown inFIG. 3, since the current density is high, namely 0.38 A/cm², the cellvoltage is not originally high.

In FIG. 3, point 5 indicates the condition before the pre-runningequivalent to the prior art. Point 6 indicates the condition after thepre-running equivalent to the prior art and before the activationtreatment equivalent to the third aspect of the invention. Point 7indicates the condition after the activation treatment equivalent to thethird aspect of the invention. As for the point 5, point 6 and point 7,as the same as the aforementioned, an analysis was carried out by anelectrochemical impedance method. FIG. 4 indicates the analytical result(Cole-Cole Plot) by an electrochemical impedance method in Embodiment A.The horizontal axis of FIG. 4 indicates a real component of impedance,and the vertical axis of FIG. 4 indicates an imaginary component ofimpedance. As shown in FIG. 4, before the pre-running equivalent to theprior art (point 5), the cell resistance is equivalent to S11−S0, andthe reaction resistance of the cathode electrode reaction is equivalentto S21−S11, and thereby the reaction resistance of the cathode electrodereaction is relatively high. In addition, as shown in FIG. 4, after thepre-running equivalent to the prior art (in which the current density isset to be 0.5 A/cm²) is carried out (point 6), the cell resistance isequivalent to S12−S0, and the reaction resistance of the electrodereaction is equivalent to S22−S12, and thereby the cell resistance andthe electrode reaction resistance is decreased. It is inferred that thewater content of the electrolyte membrane gradually increases by thepre-running equivalent to the prior art of the fuel cell.

In addition, as shown in FIG. 4, after the activation treatmentequivalent to the third aspect of the invention (the treatment forsupplying the nitrogen gas as the non-oxidant gas to the cathode) iscarried out on the fuel cell (point 7), the cell resistance isequivalent to S13−S0, and the reaction resistance of the electrodereaction is equivalent to S23−S13. It was analyzed that although thecell resistance was almost unchanged, the reaction resistance of theelectrode reaction decreased as much as ΔS. Through the above-mentionedanalytical results, the activation treatment equivalent to the thirdaspect of the invention (in which nitrogen is supplied to the cathode)is effective in lowering the reaction resistance of the electrodereaction, and it is favorable for improving the output voltage of thefuel cell.

Embodiment B

In Embodiment B, an activation treatment equivalent to the fourth aspectof the invention (in which the cathode is set to be an oxygen shortagecondition) is carried out. In Embodiment B, the activation treatment iscarried out in the oxygen shortage condition in which an oxygenutilization rate is over 100%. According to Embodiment B, when a fuelcell started, pure hydrogen gas (pressure: normal pressure) was suppliedto an anode, and at the same time, air (pressure: normal pressure) wassupplied to a cathode, and power generation running was carried out. InFIG. 5, characteristic lines V10, V11 and V12 indicate voltage, andcharacteristic lines A10, A11 and A12 indicate current. Thecharacteristic lines A10 and V10 in FIG. 5 indicate the condition justafter the starting of the power generation running of the fuel cell. Thecharacteristic lines A11 and V11 in FIG. 5 indicate the condition thatthe activation treatment equivalent to the fourth aspect of theinvention is carried out. The characteristic lines A12 and V12 in FIG. 5indicate the condition of power generation running after the activationtreatment. The plot of “◯” as shown in FIG. 5 is the mark of makingCole-Cole Plot.

As above-mentioned, in Embodiment B, the characteristic lines A11 andV11 indicate the condition that the activation treatment equivalent tothe fourth aspect of the invention is carried out. According to theactivation treatment equivalent to the fourth aspect of the invention,when the pure hydrogen gas (pressure: normal pressure) was supplied tothe anode and the air (pressure: normal pressure) was supplied to thecathode, the activation treatment was carried out. In the activationtreatment, the current density is maintained at 0.38 A/cm² as shown inthe characteristic line A11, and the cell voltage descends to around 0 v(about 0.01 v) as shown in the characteristic line V11. Since the cellvoltage is the difference of the potential between the anode and thecathode and the voltage of the anode is regarded as 0 v, the cellvoltage will be the potential of the cathode.

As for the condition before the starting of the activation treatmentequivalent to the fourth aspect of the invention (point 10) and thecondition after that (point 11), as the same as the aforementioned, ananalysis was carried out by an electrochemical impedance method. Thetime ranging from after the activation treatment to point 11 is long, inorder to confirm the continuous effect of activation effect. FIG. 6indicates the analytical result (Cole-Cole Plot). The horizontal axis ofFIG. 6 indicates the real component of impedance and the vertical axisof FIG. 6 indicates the imaginary component of impedance.

As shown in FIG. 6, before the activation treatment equivalent to thefourth aspect of the invention is carried out for the fuel cell (point10), the cell resistance is equivalent to U12−U0, and the reactionresistance of the electrode reaction is equivalent to U22−U12. Inaddition, after the activation treatment equivalent to the fourth aspectof the invention is carried out (point 11), the cell resistance isequivalent to U13−U0, and the reaction resistance of the electrodereaction is equivalent to U23−U13, and thereby it is analyzed that thereaction resistance of the electrode reaction is decreased as much asAU. According to the above-mentioned analytical result, the activationtreatment equivalent to the fourth aspect of the invention (in which thecathode is set to be an oxygen shortage condition) is effective inlowering the reaction resistance of the electrode reaction, and it isfavorable for improving the output voltage of the fuel cell.

EXAMPLES

The present invention will be hereinafter described in Example 1-Example3.

Example 1

Example 1 is equivalent to the first aspect and the second aspect of theinvention. First, 300 g of carbon black were mixed in 1000 g of water tomake water mixture. The water mixture was agitated in an agitator forthe predetermined time (10 minutes) to make agitated water. Then 250 gof tetrafluoroethylene (hereafter referred to as “PTFE”, made by DaikinIndustries Ltd.), containing the original dispersion solution with 60 wt% in concentration (trade name: POLYFLON, D1 grade), was added to theagitated water, and agitated for the predetermined time (10 minutes) toform a carbon ink.

A carbon paper (TORAYCA TGP-060, 180 μm thick, made by TORAY INDUSTRIES,INC.) was dipped into the carbon ink, and was soaked in theabove-mentioned PTFE enough to form a raw material.

Next, using a dry oven which was maintained at 80° C. to evaporate theresidual moisture contained in the raw material. After that, the PTFE inthe raw material was sintered at the temperature of 390° C. for 60minutes until the hydrophobic carbon paper was made. Then, 12 g ofplatinum-loading carbon catalyst with a platinum concentration of 46 wt% (TEC10E60E, made by Tanaka Precious Metals Industry K. K.) was fullymixed with 106 g of ionic exchange resin solution with a concentrationof 5 wt % (SS-1080, made by Asahi Kasei Corporation), 23 g of water, and23 g of isopropyl alcohol as plasticizer to form a catalytic paste.

Then, the catalytic paste was coated on a tetrafluoroethylene sheet by adoctor blade method in order that the platinum loading amount was 0.6mmg/cm², and a catalytic layer was formed. After that, the drying wascarried out. Due to this, a cathode sheet having tetrafluoroethylenesheet was formed. Platinum works as a cathode catalytic metal.

Further, an alloy-loading carbon catalyst (TEC61E54, made by TanakaPrecious Metals Industry K. K.) in which an alloy of platinum (30 wt %in loading concentration) and ruthenium (23 wt % in loadingconcentration) was loaded, which was used instead of the above-mentionedplatinum-loading carbon to form an anode sheet havingtetrafluoroethylene sheet by the same method as the aforementioned.

According to the Example, the 25 μm-thick ionic exchange membrane(Nafion 111, made by Du Pont Kabushiki Kaisha) was used as anelectrolyte membrane. The electrolyte membrane was sandwiched by thecathode sheet and the anode sheet as aforementioned. And the catalyticlayer mainly composed of platinum as a catalytic metal exists betweenthe electrolyte membrane and the cathode sheet. The catalytic layermainly composed of platinum and ruthenium as a catalytic metal existsbetween the electrolyte membrane and the anode sheet. In the conditionthat the temperature was set to be 150° C. and pressure is 10 MPa, thesetwo catalytic layers were hot-pressed for the predetermined time (1minute) to be transferred on two sides of the electrolyte membranerespectively. After that, the aforementioned tetrafluoroethylene sheetwas peeled off.

A gas diffusion layer for the cathode was set on the external side ofthe cathode catalytic layer, and a gas diffusion layer for the anode wasset on the external side of the anode catalytic layer. In the conditionthat the temperature was set to be 140° C. and the pressure was 8 Mpa,these two diffusion layers were hot-pressed for the predetermined time(3 minutes) to form a membrane electrode assembly (MEA). The membraneelectrode assembly formed a single cell.

FIG. 7 shows the conceptual diagram of a cell. As shown in FIG. 7, thecell has an electrolyte membrane 100 formed by a solid high molecularmembrane, and an anode 101 and a cathode 102 for sandwiching theelectrolyte membrane 100. The gas containing hydrogen is supplied from agas flow distribution plate 103 to the anode 101 through a flow passage104. The air is supplied from a gas distribution plate 105 to thecathode 102 through a flow passage 106. Here, the air is equivalent tothe oxidant gas containing oxygen. The gas containing hydrogen is thenatural-gas reforming simulated gas for simulating fuel gas which isused frequently in fact.

At the time of the activation treatment, in the condition of theelectrical connection formed by the anode 101 and the cathode 102 whichwere connected by a conductor 200 via a load 201, when the celltemperature was set to be 75° C., the air (oxygen utilization rate: 40%)was supplied to the cathode 102 and the natural-gas reforming simulatedgas containing 10 ppm of CO (hydrogen utilization rate: 90%) wassupplied to the anode 101 under the normal pressure, and in which thepotential of the cathode 102 was set to be 0.05 v (around 0 v) andmaintained for 5 minutes; thus the activation treatment was carried out.In the activation treatment, the anode 101 was used as a negativeelectrode and the cathode 102 was used as a positive electrode. However,in the activation treatment of Example 1, the oxygen utilization rate ofthe cathode 102 can set to be 50% or less. This aims to suppressflooding. In the Example, the potential was set to be 0.05 v andmaintained at a constant potential voltage by a potentiostat stabilizer.After the activation treatment, when the normal generation running wascarried out at 0.38 A/cm², as shown in Table 1, the high cell voltage as0.725 v was obtained and the cell voltage was improved as compared withthe condition before the activation treatment.

The present inventor infers that the reason for the improvement of thecell voltage output in the activation treatment of Example 1 is asfollows. In the activation treatment, because the power generation iscarried out with the cell voltage of around 0 v (0.05 v), it is inferredthat the electrochemical reduction reaction about oxygen is generated onthe cathode 102, and at the same time, the electrochemical reductionreaction about other substances is generated. Namely, the reductionreaction of platinum oxide or adsorbed elements (including foreignsubstances) is generated on the surface of the platinum constituting thecathode 102, so that the active sites of platinum are opened at thecathode 102, to activate the platinum, so that the reaction resistanceof the electrode reaction is lowered, and the cell voltage output ismuch improved than that before the activation treatment.

Example 2

Example 2 is equivalent to the first aspect and the third aspect of theinvention (in which non-oxidant gas is introduced into the cathode). Amembrane electrode assembly (MEA) made in the Example 1 was used to forma single cell. As shown in FIG. 8, the single cell has an electrolytemembrane 100 formed by a polymer electrolyte membrane and an anode 101and a cathode 102 for sandwiching the electrolyte membrane 100. The gascontaining hydrogen is supplied from a gas distribution plate 103 to theanode 101 through a flow passage 104. The air is supplied from a gasdistribution plate 105 to the cathode 102 through a flow passage 106.Moreover, the cathode 102 is connected with a flow passage 150, whichsupplies the nitrogen gas (non-oxidant gas) to the cathode.

And, at the time of the activation treatment, in the condition of theelectrical connection formed by the anode 101 and the cathode 102 whichwere connected with the conductor 200 via a load 201, when the celltemperature was set to be 75° C., nitrogen gas (purge gas) was suppliedto the cathode 102 from the flow passage 150 while the air supply wasstopped, and the natural-gas reforming simulated gas (hydrogenutilization rate: 90%) containing 10 ppm of CO was supplied to the anode101 under the normal pressure respectively. Then, in the condition ofthe connection with electronic load, the activation treatment wascarried out for 5 minutes, while the current density was maintained at0.38 A/cm², and the electric potential of the cathode 102 was set about0.01 v. The aforementioned natural-gas reforming simulated gas is thegas containing hydrogen. In the Example, the potentiostat stabilizer isnot used because of the controlled current running.

After the above-mentioned activation treatment, the air (oxygenutilization rate: 40%) was supplied to the cathode 102 through a flowpassage 106, and the natural-gas reforming simulated gas was supplied tothe anode 101, and the normal power generation running was conducted at0.38 A/cm². As shown in Table 1, the high cell voltage as 0.724 v wasgained, and the cell voltage was improved. The oxygen utilization rate(%) means (the actual oxygen amount used in power generation/the oxygenamount supplied to the fuel cell)×100%. The hydrogen utilization rate(%) means (the actual hydrogen amount used in power generation/thehydrogen amount supplied to the fuel cell)×100%.

In a model test, the specific oxygen utilization rate (%) was calculatedas follows. Here, electrode area is S (cm²), current density is i(A/cm²) and cell number is n. The supply amount of oxygen in unit time N(mol/sec), which is necessary for power generation of the fuel cell isas follows.N=(S×i×n)/4F.

In this model test, S is 59 cm²; i is 0.38 A/cm² and n is 15.

The necessary supply amount of oxygen N (mol/sec) isN=(S×i×n)/4F=(59×0.38×15)/(4×96500)=0.000871 (mol/sec).

In the case of air, N/0.21=0.00415 (mol/sec).

When it is converted into air volume, 0.00415×22.4=0.0929liter/sec.=5.57 liters/minute.

So, in the model test, if 5.57 liters/minute of air is supplied to thecathode, the oxygen utilization rate will be 100%. If 13.9litters/minute of air is supplied to the cathode, the oxygen utilizationrate will be 40%. Namely, (5.57/13.9)×100%=40%.

The present inventor infers that the reason for the improvement of thecell voltage output in the activation treatment of Example 2 is asfollows. In the activation treatment, because the nitrogen gas (purgegas) is supplied to the cathode 102 instead of air, it is inferred thatthe oxygen shortage condition is actively generated on the cathode 102,and the electrochemical reduction reaction of other substances isconducted more actively on the cathode 102 than the electrochemicalreduction reaction of oxygen. Namely, it is likely that the product suchas platinum oxide and so on is generated on platinum constituting acatalyst in the cathode 102, or substances are adsorbed on the catalyticmetal. Therefore, the reduction reaction of the product such as platinumoxide and so on, or the reduction reaction of adsorbed elements occursby the activation treatment, so that the active sites of platinum ascatalyst at the cathode 102 are opened to activate the platinum, andthereby the reaction resistance of the electrode reaction on the cathodeis lowered and the cell voltage output is improved. As above-mentioned,in the activation treatment in which the nitrogen gas and so on issupplied to the cathode 102, the gas containing hydrogen is supplied tothe anode 101, as the nitrogen gas can be distributed evenly to thecathode 102 of each cell even when the number of cells constituting thefuel cell is plural, so it is possible to decrease the uneven activationtreatment between all the cells.

Example 3

Example 3 is equivalent to the first aspect and the fourth aspect of theinvention (in which the cathode is forcibly set to an oxygen shortagecondition). A membrane electrode assembly (MEA) made in the Example 1was used to form a single cell. FIG. 7 is applied to Example 3. As shownin FIG. 7, the single cell has an electrolyte membrane 100 formed by apolymer electrolyte membrane and an anode 101 and a cathode 102 forsandwiching the electrolyte membrane 100. The gas containing hydrogen issupplied from a gas distribution plate 103 to the anode 101 through aflow passage 104. The air is supplied from a gas distribution plate 105to the cathode 102 through a flow passage 106.

And, in the condition of the electrical connection formed by the anode101 and the cathode 102 which were connected with the conductor 200 viaa load 201, when the cell temperature was set to be 75° C., the air(oxygen utilization rate: 200%) was supplied to the cathode 102, andnatural-gas reforming simulated gas (hydrogen utilization rate: 90%)containing 10 ppm of CO was supplied to the anode 101 under the normalpressure respectively, and while the current density was maintained at0.38 A/cm², and the potential of the cathode 102 was set to be around0.01 v, the activation treatment was carried out for 5 minutes. In theExample, the potentiostat stabilizer is not used because of thecontrolled current running.

According to the Example, as above-mentioned, the oxygen utilizationrate of the cathode 102 is 200%. This means that the cathode 102 is setto an oxygen shortage condition, and that the electrochemical reductionreaction is conducted at the cathode 102 more than the electrochemicalreduction reaction based on the amount of the oxygen in the air suppliedto the cathode 102.

It is inferred that: (1) the reduction reaction of the product such asplatinum oxide and the like on platinum constituting a catalytic metal,(2) the electrochemical reduction reaction of adsorbed elements on thecatalytic metal, or (3) the electrochemical reduction reaction ofprotons (H⁺) penetrated from the anode 101 to the cathode 102 throughthe electrolyte membrane 100—these reduction reactions are generated atthe cathode 102. After the above-mentioned activation treatment wascarried out, when the oxygen utilization rate was changed to 40%, andthe normal power generation running was carried out at 0.38 A/cm², asshown in Table 1, the high cell voltage as 0.725 v could be obtained andthe cell voltage was improved in comparison with the cell voltage beforethe activation treatment.

The present inventor infers that the reason for the improvement of thecell voltage output in the activation treatment of Example 3 is asfollows. In the activation treatment, because the oxygen shortagecondition is forcibly maintained on the cathode 102, the electrochemicalreduction reaction concerning oxygen is conducted on the cathode 102,and at the same time, the electrochemical reduction reaction of othersubstances is conducted actively. Namely, it is likely that the productsuch as platinum oxide and so on or adsorbed elements is generated onplatinum constituting a catalyst in the cathode 102. It is inferred thatthe electrochemical reduction reaction of the product such as platinumoxide and the like on platinum constituting a catalytic metal in thecathode 102, and the electrochemical reduction reaction of adsorbedelements on the catalytic metal is conducted, so that the active sitesof platinum as catalyst are opened, and thereby the reaction resistanceof the electrode reaction is lowered and the cell voltage output isimproved.

Comparative Example 1

A single cell was formed by the membrane electrode assembly (MEA) madein Example 1. In the condition that the cell temperature was set to be75° C., the air (oxygen utilization rate: 40%) was supplied to a cathode102, and the natural-gas reforming simulated gas (hydrogen utilizationrate: 90%) containing 10 ppm of CO was supplied to an anode 101 underthe normal pressure, respectively. In this case, as shown in Table 1,the cell voltage output of 0.670 v was gained, which was lower than thatin Examples 1-3.

Comparative Example 2

A single cell was formed by the membrane electrode assembly (MEA) madein Example 1. In the condition that the cell temperature was set to be75° C., the air (oxygen utilization rate: 40%) was supplied to a cathode102, and the natural-gas reforming simulated gas (hydrogen utilizationrate: 90%) (gas simulating the reformed gas which has been reformed fromthe natural-gas) containing 10 ppm of CO was supplied to an anode 101under the normal pressure, respectively, and then, the pre-running wascarried out at 0.50 A/cm2 for 2 hours. The pre-running is equivalent tothat of the prior art. The potential of the cathode in the pre-runningof Comparative Example 2 was 0.55-0.66 v. After that, when the normalgeneration running was carried out at 0.38 A/cm², as shown in Table 1,the cell voltage output of 0.685 v was gained, which was lower than thatin Examples 1-3. As known from the above-mentioned result, according tothe solid high polymer electrolyte type fuel cell in Examples 1-3 whichcarries out the activation treatment, the cell voltage output wassuperior to that of the solid high polymer electrolyte type fuel cell incomparative Examples 1 and 2.

TABLE 1 Comparative Comparative Example 1 Example 2 Example 3 Example 1Example 2 Cell 0.725 0.724 0.725 0.670 0.685 Voltage V

(Surface of Platinum Electrode)

According to the prior art documents, the following reactions occur onthe surface of the platinum electrode:[PtCl₄]²⁻+2e ⁻=Pt+4Cl⁻

-   -   standard oxidation reduction potential 0.758 v        PtO+2H⁺+2e ⁻=Pt+H₂O    -   standard oxidation reduction potential=0.98 v        Pt²+2e ⁻=Pt    -   standard oxidation reduction potential=1.188 v

Here, the reaction in which the oxidation reduction potential is clearis shown, but the same applies to the reaction with other substances.The standard oxidation reduction potential is the balance potential whenthe concentration (activity) of oxidation electrode and reductionelectrode is set at 1. According to the formula of Nernest, the balancepotential of oxidation reduction reaction ofM ^(n+) +ne ⁻ =M

shows the following relation between the concentration of each oxidationelements C_(M) ^(n+) and reduction elements C_(M)E=E _(o)+(RT/nF)1n(C _(M) ^(n+) /C _(M))

In the equation: E_(o) is the standard oxidation reduction potential, Ris the gas constant, T is the absolute temperature, F is the Faradayconstant. Therefore, when the concentration of oxidation elements islow, the balance potential is lower than the standard oxidationreduction potential, which means the oxidation reduction reaction is tobe generated a lower potential. On the surface of the fuel cell, becauseit is guessed that the concentration of the oxidation elements ofplatinum and other substances is less than 1 in fact, it can be expectedthat the balance potential is lower than the standard oxidationreduction potential.

According to the prior art document, the following absorption isoccurred on the surface of the platinum electrode at a range ofelectrode potential of 0.7-1.0 v:Pt+H₂O+e ⁻=Pt.OH+H⁺

When the air (oxygen) exists on the surface of the platinum electrode,½O₂+2e ⁻+2H⁺═H₂O

standard oxidation reduction potential=1.229 v

The platinum electrode potential is regulated by the aforementionedoxidation reduction reaction of oxygen. In the manufacturing stage ofthe fuel cell, in the discharge stage before use, or depending on theelectrode potential in a power generation, the platinum oxidation orsubstance adsorption can be generated on the surface of platinumelectrode. As a result, the active sites on the surface of the platinumare reduced. It is necessary to pull down the platinum electrodepotential below the balance potential of these reactions (for examplearound 0 v) once, in order to reduce these products or to separate theseadsorbed elements from the platinum, and then, to open the occupiedactive sites. It is inferred that when the platinum electrode potentialcomes to lower than the balance potential thereof, the reduction orseparation speed is accelerated, and the active sites on the platinumsurface are opened to activate the platinum. There are two main methodsto pull down the platinum electrode potential: one is to raise the speedof oxygen reduction reaction (electrode current density) to enlarge apolarization, and the other is to remove oxygen to regulate the platinumelectrode potential by other oxidation-reduction reactions.

Other Examples

According to Example 2, at the time of the activation treatment, asshown in FIG. 8, the gas containing hydrogen is supplied to an anode 101through a flow passage 104, and at the same time, the nitrogen gas asnon-oxidant gas is supplied to a cathode 102 through a flow passage 150.However, without restricting to this, another example may be carriedout—while the gas containing hydrogen is supplied to the anode 101through the flow passage 104, mixed gas (non-oxidant gas) containing adiluted concentration of hydrogen with respect to the gas supplied tothe anode 101 can be supplied to the cathode 102.

In this case, as shown in FIG. 8, a flow passage 170 in which hydrogengas flows and a flow passage 171 in which the diluting gas such asnitrogen gas flows can be communicated with a flow distribution plate105 of the cathode 102. Then, the mixed gas in which the hydrogen gasfrom the flow passage 170 is mixed with the diluting gas of the flowpassage 171 is supplied to the cathode 102.

FIG. 9 indicates another form in Example 1, 2 and 3. According to these,as shown in FIG. 9, a small load 205 whose electric resistance isrelatively smaller than that of a load 203 which is driven by normalpower generation reaction. The small load 205 is set between the anode101 and the cathode 102, and is electrically parallel to the load 203.In addition, a switching element 300 is mounted. The switching element300 has a function for switching over between a first conductive path202 connecting the load 203 driven by normal generation reaction and asecond conductive path 206 connecting the small load 205 which hasrelatively small electric resistance.

At the time of the normal generation running, when the first conductivepath 202 connecting the load 203 driven by normal generation reaction isswitched on by the switching element 300, and the second conductive path206 connecting the small load 205 is switched off. On the contrary, atthe above activation treatment, the first conductive path 202 connectingthe load 203 is switched off by the switching element 300, and at thesame time, the second conductive path 206 connecting the load 205 isswitched on. Therefore, in the above-mentioned activation treatment,because the electrons (e⁻) generated in oxidation reaction on the anode101 move to the cathode 102 through the second conductive path 206having the small load 205 whose electric resistance is relatively small,so that this is advantageous for accelerating the activation treatmentspeed and shortening the time for activation treatment on the cathode102.

When the gas containing oxygen is supplied to the cathode 102, becausethe oxygen reduction reaction occurs on the cathode 102 by priority.Thus, allowing a lot of current to flow through the small load 205 isespecially advantageous for raising the reduction reaction on thecathode 102, and for raising the reduction reaction of the platinumoxide and adsorbed elements on the catalytic metal. As the small load205, one having small electric resistance can be set, and the resistanceof the conductive line forming the second conductive path 206 can be asubstitution.

FIG. 10 indicates the other form in Examples 1, 2 and 3. According tothese, as shown in FIG. 10, a battery 207 is set on a conductive path208 as an external supply means for supplying electrons to the cathode102. It is possible that a negative electrode of the battery 207 isconnected to the cathode 102 while a positive electrode of the battery207 is connected to the anode 101. The conductive path 208 is setparallel to the conductive paths 202 and 206. In the activationtreatment, the potential of the cathode 102 is sometimes lower than thatof the anode 101, depending on conditions. In this case, as electronenergy potential of the cathode 102 is higher than that of the anode101, it is likely that the electrons supplied from the anode 101 to thecathode 102 are limited, and then, the reduction reaction based on theactivation treatment is limited on the cathode 102. So, the activationtreatment can be carried out by switching on the switching element 300,and supplying electrons forcibly from the external battery 207 to thecathode 102 of the fuel cell. In this case, the electrochemicalreduction reaction (reduction reaction of catalytic oxide and adsorbedelements) on the cathode 102 is secured, and the active sites of thecatalytic metal on the cathode 102 are opened. The battery 207 can bereplaced with a capacitor.

Applicable Example

FIG. 11 indicates an applicable Example. As shown in FIG. 11, a powergenerating system of the fuel cell has a polymer electrolyte fuel cell301 formed by a stack of multi-layered cells, a first conductive path202 having a load 203 driven by the power generation reaction of thefuel cell 301, a second conductive path 206 connecting a load 205, whoseelectric resistance is relatively smaller than that of the load 203, aswitching element 300 used to switch on or switch off the secondconductive path 206, a second power supply 302 functioned as anauxiliary power, and a switching element 303 which switches a powersupply for driving the load 203.

The switching element 303 has a first switching element 304 forconnecting or disconnecting the fuel cell 301 with the load 203, and asecond switching element 305 for connecting or disconnecting the secondpower supply 302 with the load 203.

In the aspect of normal power generation running, when the switchingelement 300 is switched off, the first switching element 304 is switchedon, and at the same time, the second switching element 305 is switchedoff. Therefore, the load 203 and the fuel cell 301 are in electricalconnection, and the load 203 and the second power supply 302 are indisconnection. As a result, the load 203 is driven by the powergeneration of the fuel cell 301. The second power supply 302 can use thecommercial power supply as direct current, or use the second fuel cell.

In addition, when the power generating performance of the fuel cell 301is degraded, the activation aspect of the above-mentioned activationtreatment is carried out on the fuel cell 301. When the activationtreatment mode is carried out on the fuel cell 301, the second switchingelement 305 is switched on, and at the same time, the first switchingelement 304 is switched off. Therefore, the load 203 and the fuel cell301 are in disconnection, and at the same time, the load 203 and thesecond power supply 302 are in electrical connection. Due to this, theload 203 is driven by the second power supply 302, not by the fuel cell301. In the activation treatment as above-mentioned, the switchingelement 300 is switched on, and the activation treatment in Examples 1-3is carried out, and the cell voltage output of the fuel cell 301 isrestored. When the cell voltage output is restored, the switchingelement 300 and the second switching element 305 are switched off again,and the first switching element 304 is switched on, to change to themode of the normal generation running. The present invention is notlimited to the Examples and applicable Example as above-mentioned andshown in Figures, and it can be carried out by making appropriatemodifications within the range of not deviating from the main points.

POSSIBILITY OF INDUSTRIAL APPLICATION

A polymer electrolyte fuel cell in the present invention can be usedextensively to a fuel cell power generation system for vehicles,emplacement, electrical equipment, electronic equipment and so on.

1. An activation method for preparing a polymer electrolyte fuel cellbefore the fuel cell generates electricity including a membraneelectrode assembly that includes an electrolyte membrane formed by apolymer electrolyte membrane, and an anode and a cathode each carrying arespective catalytic metal and sandwiching said electrolyte membrane,wherein said anode and said cathode are in electrical connection; and anactivation treatment opens an active site of said catalytic metal ofsaid cathode by supplying gas containing hydrogen to said anode,supplying gas containing oxygen to said cathode, and setting saidcathode in an oxygen shortage condition in which an oxygen utilizationrate is over 100%, wherein the oxygen utilization rate corresponds to aratio of an oxygen amount actually utilized in power generation to anamount of oxygen supplied to the polymer electrolyte fuel cell.
 2. Theactivation method according to claim 1, wherein said activationtreatment is carried out by supplying electrons forcibly to said cathodefrom an external environment.
 3. The activation method according toclaim 2, wherein said external environment is a battery.
 4. Theactivation method according to claim 1, wherein said activationtreatment is carried out for maintaining potential of said cathode at0.5v or less.
 5. The activation method according to claim 1, wherein asecond conductive path whose electric resistance is relatively lowerthan that of a first conductive path in normal operation for generationof electricity, and said activation treatment is carried out in thecondition that said anode and said cathode are in electrical connectionby way of said second conductive path.
 6. The activation methodaccording to claim 1, wherein the oxygen utilization rate is set to 200%during the activation treatment.
 7. The activation method according toclaim 6, further comprising: a normal power generation running mode thatincludes an oxygen utilization rate of 40%.